A statistic credited to a defensive players when physical contact with a quarterback causes him to be ruled down by contact behind the line of scrimmage. In order for a sack to be recorded, it must be obvious that the quarterback either intends to throw a forward pass, or is still in the "pocket" without a clear objective for the play.
More about Team Sacks and Sack Yards
Total team sacks isn't always indicative of a good defense, it also needs to be taken into context with total plays that the defense has been on the field. For example a team might have a couple good defensive ends that can get to the quarterback, but the rest of their defense is not good and therefore spending more time and plays on the field.
The worst teams are those with a lot of defensive snaps and low number of sacks.